$300 for a return J ticket Japan-USA is quite good!!!
As said above, such an obvious error fare has little chance to be honored when there are 1,000 tickets issued.
CX had an error fare from Vietnam at $600+ in J and 1+k in F. Surprisingly they honored it although a large number of tickets were sold. That makes HK guys, like me, very upset as award or cheap J availability HKG-Vietnam has disappeared and HKG-USA availability in premium cabin has dried up considerably.